First, the premium for a Part C MA plan is
in addition to the Original Medicare Part B $134 premium, not instead of. Some Part C MA plans are $0 premium, but are not available everywhere. My Part C premium of $120 is shown as $67 toward Part B medical coverage, and $53 toward Part D Rx coverage.
See
https://www.medicare.gov/sign-up-cha...#collapse-3135 under Step 1 > Choose MA Plan > How much does it cost
"You usually pay a monthly premium for your MA Plan (in addition to your monthly Part B premium)"
Second, while you may have SEP of 8 months to enroll in Original Medicare Parts A & B, you may only have an SEP of 2 months to enroll in a Part C MA plan
See page 8 of
https://www.medicare.gov/Pubs/pdf/11...e-Part-C-D.pdf under Leaving Employer
"You leave coverage from your employer or union > Your chance to join lasts for 2 full months after the month your coverage ends."